When a mink on a large farm in Galicia, a region in northwestern Spain, died in October 2022, veterinarians initially thought the culprit was SARS-CoV-2, which has struck mink farms in many other countries. But laboratory tests revealed something more frightening: a deadly bird flu virus called H5N1. The authorities immediately placed workers on the farm under quarantine restrictions. More than 50,000 mink were killed at the facility and their carcasses destroyed.
None of the farm workers were infected. But the incident, described in paper i Eurosurveillance last week, there have long been fears that H5N1 could trigger a human pandemic. The virus is not known to spread well between mammals; humans almost always catch it from infected birds, not from each other. But now, H5N1 appears to have spread through a tightly packed mammal population and acquired at least one mutation that favors mammal-to-mammal spread. Virologists warn that H5N1, now roaming through birds around the world, could invade other mink farms and become even more transmissible.
“This is very worrying,” says Tom Peacock, a virologist at Imperial College London. “This is a clear mechanism for the initiation of the H5 pandemic.” Isabella Monne, a veterinary researcher at the European Union Reference Laboratory for Avian Influenza in Italy, where the samples from Spain were sequenced, calls the decision a “warning bell”.
H5N1 was first detected at a goose farm in China in 1996. A large poultry outbreak in Hong Kong in 1997 led to the first documented deaths in humans and prompted the first fears of a pandemic. Around 2005, the virus spilled into migratory birds, which has since spread around the world in several large waves. A new version called 184.108.40.206ba that emerged in 2020 has spread faster and further than any predecessor, dealing massive blows to the poultry industry in Europe and North America before arriving in Central and South America with fall 2022. “This virus seems to be more suited to each bird than any other bird,” says Richard Webby, a flu researcher at St. John’s Children’s Research Hospital. Jude.
Because the receptors that the virus binds to in the upper airways of birds are not as common in the upper airways of mammals, H5N1 infects mammals more. But this time many species of mammals are infected, including foxes, cats, ferrets, seals and dolphins, probably through contact with infected birds. On January 17, Montana authorities said that three young grizzly bears euthanized in the fall had also become very ill infected with H5N1. People are caught up in it, too. So far there have been six confirmed human infections in the current global wave, including one death.
There are some signs that 220.127.116.11b is less pathogenic in humans than earlier versions, which killed about half of those infected, says Thomas Mettenleiter, head of the Friedrich Loeffler Institute. “Of course that can also be bad news, because it can make it easier for the virus to start spreading under the radar, giving it more opportunity to evolve,” he says. The more often the virus infects a mammal, the greater the risk, says Webby. “It’s a numbers game.”
There have been several past reports of bird flu outbreaks on mink farms in China, but there is no clear evidence that the virus was spread between the animals. In the Spanish outbreak, there appears to be little doubt. In theory, all the sick animals could have picked up the virus from their feed, including poultry by-products, but no H5N1 outbreaks have been reported in the region where the poultry farms and slaughterhouses that supply the feed are located. And the virus spreads from pen to pen as would be expected if it were transmitted between minks. The chain of infections may have started after one animal caught a sick bird and dragged it into its cage, says Thijs Kuiken, a veterinary pathologist at the Erasmus University Medical Center.
It is not known how easily the virus found in Spain could infect people – or spread between them. Virus samples sequenced from four minks show several changes compared to the bird virus, including T271A, a mutation in the gene for an enzyme, polymerase. The change, also seen in viral samples from other infected mammals, is known to help H5N1 replicate better in mammalian tissues. E627K, another worrisome mutation in the polymerase gene, did not occur, however, and the gene for hemagglutinin—a protein on the viral surface that attaches to the host plant—was unchanged, Peacock says. “We might still get lucky with this one.”
Monne says her team and others are studying the properties of the mink virus and the effects of the mutations it has accumulated. Among other things, they want to study how well the virus is transmitted through close contact between animals. “We also plan to do aerosol transmission studies,” she says.
The outbreak once again highlights the risks associated with mink farming. SARS-CoV-2, introduced to farms by humans, spread like wildfire among the animals but was also passed back to their keepers, with researchers concerned that the mink industry could be the source permanent infections and a breeding ground for genetic variation. The Netherlands, which had already decided to end mink farming by 2024 for ethical reasons, closed all remaining farms in 2021. Denmark killed all mink in the country in 2020, but a ban on mink farming expired at the beginning of this year.
Farms are just as big a threat when it comes to H5N1, Kuiken says. Most of the mammal species infected with the virus so far are wild predators and scavengers that feed on infected birds — “single animals, or animals that live in small families,” he says. They are unlikely to spread the virus far or infect people. On mink farms, thousands of such individual carnivores are forced to live together, creating ideal conditions for the avian virus to adapt to mammals. “It’s a human construct,” says Kuiken.
At the very least, biosecurity measures on mink farms must be tightened, says Monne. Farm workers should wear masks and take other measures to prevent infection, and farms should reduce the risk of accidentally introducing H5N1. “They should carefully keep the animals far away from wild birds.” peacock says maybe it’s time to end mink farming. “That this is happening in Europe today, and after COVID-19, makes my head spin,” he says. “It’s a bit of an existential threat.”